This file has a confederate Musketeer fusilier and Union musketeer and fusilier. The musketeers (just one of them) need to be renamed to musketeer_amer, and be sure to make a copy of the original so you don't lose it. You also must make a copy of highlander.BH3 and rename it to musketeer_amer.BH3, and again be sure to make a copy of the original.
The fusiliers are to be renamed to fusilier_american. Also make a copy of fusilier.BH3 and rename it to fusilier_american.BH3. Be sure to make copies of the original and you can only have one of each in the game, unless you replace a european unit or something.
I havn't looked into the new way of adding files with Patch 3, but this way still works. Hopefully, there will be a package containing an assortment of musketeers. Have fun!